1. Field of the Invention
This invention relates generally to storage, separation and analysis of ions according to mass-to-charge ratios of charged particles and charged particles derived from atoms, molecules, particles, sub-atomic particles and ions. More specifically, the present invention is a device for performing mass spectrometry using a virtual ion trap, wherein the aspect of being virtual is in reference to the elimination of electrodes to thereby remove physical obstructions that result in more open access to a trapping volume.
2. Description of Related Art
Mass spectrometry (MS) is one of the most important techniques used by analytical chemists for identifying and quantifying trace levels of chemical elements and compounds in environmental and biological samples. Accordingly, MS can be performed as an independent process. However, MS becomes more powerful when coupled to separation techniques such as gas chromatography, liquid chromatography, capillary electrophoresis, and ion mobility spectrometry.
In MS, ions are separated according to their mass-to-charge ratios in various fields, including magnetic, electric, and quadrupole. One type of quadrupole mass spectrometer is an ion trap. Several variations of ion trap mass spectrometers have been developed for analyzing ions. These devices include hyperbolic configurations, as well as Paul, dynamic Penning, and dynamic Kingdon traps. In all of these devices, ions are collected and held in a trap by an oscillating electric field. Changes in the properties of the oscillating electric field, such as amplitude, frequency, superposition of an AC or DC field and other methods can be used to cause the ions to be selectively ejected from the trap to a detector according to the mass-to-charge ratios of the ions.
Mass spectrometers are mainly classified by reference to a mass analyzer that is used. These mass analyzers included magnetic and electric sector, ion cyclotron resonance (ICR), quadrupole, time-of-flight (TOF), and radio frequency (RF) ion trap.
Each of these mass analyzers has its own advantages and disadvantages. For example, sector and ICR instruments are known for their high mass resolution, TOF for its speed, and quadrupoles and ion traps for their simplicity and small size. ICR and sector instruments are typically large and complex to operate, and as with TOF, require high vacuum, while quadrupoles and ion traps operate at higher pressures but deliver lower mass resolution. Most analytical problems can be solved using lower performance instruments. Therefore, quadrupole and ion trap mass spectrometers, that are significantly less expensive, are used ubiquitously in the industry.
A mass spectrometer is comprised of an ion source that prepares ions for analysis, an analyzer that separates the ions according to their mass-to-charge ratios, and a detector that amplifies the ion signals for recording and storage by a data system.
It was noted above that one particular advantage of ion trap mass spectrometers is that these devices typically do not require as high a vacuum within which to operate as other types of mass spectrometers. In fact, the performance of the ion trap mass spectrometer can be improved due to collisional dampening effects due to the background gas that is present. Ion trap mass spectrometers typically operate best at pressures in the mTorr range.
It is also observed that the smaller the ion trap, the higher the possible operating pressure. This is an important advantage for portable and handheld instruments, not only because of the reduced size of the ion trap, the electronics and power requirements, but also because of the reduced size of the vacuum pump that must be used.
It is important to also note that there has been considerable interest in reducing the size of ion trap mass spectrometers for portable and handheld use. Disadvantageously, a major problem with reducing the size of the ion trap is that machining tolerances become more critical at small sizes while trying to retain good ion trap resolution. One example of a small ion trap was reported by a research group at Oak Ridge. The device is basically a miniaturized version of a cylindrical ion trap with no real changes in the structure, but just the size.
It is also noted that the capacity for trapping ions is another issue when dealing with a small ion trap because of the issue of space-charge repulsion of particles within the trap.
Accordingly, what is needed is an ion trap that can be easily miniaturized without compromising resolution of the MS, provide easier access to the trapping volume, maximize space within a trapping volume, and meet manufacturing tolerances more easily than prior art machining techniques.